I dont know if you can reliably control the volume for 2 zones. You should theoretically be able to set the startup volume then have the up/down volume commands both on a single button (up or down) control the 2 zones together but there is always a risk that one will get louder than the other especially if you try and do one crestron stereo zone and 1 surround zone. Even if you have serial control of the surround receiver and the ability to set the volume at startup it will ramp up and down differently.

This screenshot shows an easy way to give the client "party mode" using global presets for your macros. Page flip into this menu from the home page. If you want to get back to it after selecting control source you have to go home and page flip into it again.